How much and how often do i water my babies which are @ APPROX 4wks old?

I have about four nearly 1.5 feet tall and are looking good so far. I am running a 600 watt bulb with the adequate wall lining. My question is how much water do I give them and how often. I have read many different suggestions but at this stage am seeking the most or closest to the most accurate facts regarding this matter from all those connoisseurs out there that have the wisdom i am seeking. All your help will greatly appreciated and very helpful.